1. Field of the Invention
The present invention relates to an implantable cardiac stimulating device of the type having the capability to adapt the AV-delay depending on different operating conditions of the cardiac stimulating device.
2. Description of the Prior Art
Under healthy heart conditions an exercise-induced increase in heart rate is followed by a shortening of the heart's physiologic P-R interval.
In pacemaker technology the term AV-delay is used for AV sequential pacing and the term PV-delay is used for sensed P-waves followed by ventricular stimulations synchronized to the sensed P-wave.
Hence, among the approaches to provide physiological pacing in modern DDD pulse generators, an algorithm for stimulation rate-correlated shortening of the AV/PV delay is used. While this is physiological during exercise and of benefit in DDD/VDD pacing at higher rates, the shortening is, as first understood by the inventor herein, less appropriate during periods of high stimulation rate at rest. This situation might occur during a situation referred to as atrial overdrive.
Atrial overdrive is used to suppress spontaneous atrial activity in patients who suffer from paroxysmal atrial tachyarrhythmias. Atrial overdrive might be implemented as dynamic overdrive which means that the pulse generator normally provides a stimulation rate that is slightly above the underlying, possibly varying, intrinsic rate.
One example of overdrive pacing is described in U.S. Pat. No. 4,503,857, which discloses the concept of dynamic overdrive in order to achieve a high percentage of pacing at a rate only slightly higher than the underlying intrinsic rate.
The article “Use of flywheel, Automatic underdrive and dynamic overdrive in atrial pacers” by C. Barnay and J. L. Medvedowski in the book “Cardiac pacemakers: Diagnostic options, dual chamber pacing, rate responsive pacing, antitachycardia pacing” provides a description of the concept of dynamic overdrive of the atria for the purpose of suppressing atrial tachyarrhythmias. The book is edited by Behrenbeck. D. W. and published by Darmstadt, Steinkopf, Springer Verlag, New York, 1985.
The algorithm disclosed in the two documents mentioned above for implementing dynamic overdrive is that if there has been an atrial inhibition the stimulation rate is increased with a predetermined step. This algorithm for increase of the stimulation rate is applied as long as there are atrial inhibitions. When 100% pacing is achieved after increasing the stimulation rate in one or several steps the pacing rate is slowly decreased until an atrial inhibition is detected in which case the stimulation rate is increased again by a predetermined step as described above. If the patient has arrhythmic tendencies such as frequent PAC's (Premature Atrial Contraction) this may result in a stimulating rate that is significantly higher than the optimal physiologic rate. If no inhibitions occur the pacemaker will stimulate at its base rate or lower rate. If the pacemaker is of the rate responsive type the pacemaker will operate at its sensor indicated rate. In the two last cases the pacemaker will overdrive the heart but it will not operate in dynamic overdrive.
None of the above documents, however, discloses anything regarding the problem with the AV-interval shortening during nonphysiological dynamic overdrive pacing for tachyarrhythmia suppression.